The present invention relates to an innovative drive unit for traction carriages, such as for drawbenches.
Generally, traction carriages are two in number and are alternately moved in a to-and-fro synchronized movement by mechanical drive units.
Drive units are known which are provided with axial cams in head-on contact with pusher rollers integral with the carriages. These cams must bear important efforts and are therefore very expensive due to their manufacture complexity and at all events they have a restricted duration of life because it is impossible to make them with appropriate surface hardness, owing to the substantial impossibility of lubricating them and to the movement dynamics generating a continuous sliding between races and rollers, since the relative peripheral speeds are forcedly different.
A further disadvantage of this solution is that the entire structure is under great stress, due to the tilting torques from the arm existing between the thrust axis of the rollers on the carriages and the traction axis of the carriages, and to the transverse components arising from the angle of inclination of the cam.
In addition, in order to increase the work speed while keeping the forces of inertia generated by the reciprocating motion of the carriages to acceptable values, the only solution consists in increasing the carriage stroke. With the above mechanical operation such a result can be achieved only if the cam diameter and/or the inclination of the cam races is increased, which however brings about an augmentation of all problems relating to costs and wear and also further problems due to an excessive bulkiness.
In order to obviate all problems present when axial cams are adopted, a drive unit has been proposed, as described in EP 0 255 740 for example, in which two radial cams are employed.
Resting on said radial cams are roller followers that are each operated by a rack engaging with a gear axially integral with a pinion, in turn engaging with a further rack integral with the carriage. In this manner, a ratio multiplying the cam stroke is achieved. This solution eliminates some of the problems connected with the use of axial cams, but on the other hand it is expensive and, due to clear mechanical and dimensional reasons, it does not allow the carriage stroke to be increased in a substantial manner.
The general object of the present invention is to obviate the above mentioned drawbacks by providing a drive unit for traction carriages enabling the carriages to carry out a stroke of large width without taking too much place, while ensuring the absence of elements submitted to excessive wear, and at the same time reduce costs and the elimination of complicate manufacture workings.